What is the $pH$ value of a basic aqueous solution?

What is the concentration of $[OH^-]$ in distilled water at $298 \; K$?

Three acidic solutions $A, B$ and $C$ have $pH = 0, 3$ and $5$ respectively.
$(i)$ Which solution has the highest concentration of $H^{+}$ ions?
$(ii)$ Which solution has the lowest concentration of $H^{+}$ ions?

Fresh milk has a $pH$ of $6$. How do you think the $pH$ will change as it turns into curd? Explain your answer.

The $pH$ of a water sample collected from a river was found to be acidic in the range of $3.5 - 4.5$. On the banks of the river were several factories that were discharging effluents into the river. The effluents of which one of the following factories is the most likely cause for lowering the $pH$ of river water?

Which solution will be basic?